We quantify this by calculating the factor by which the disc component
underpredicts the observed flux in the UVW1 filter ('UV excess':
GDP09) in each spectrum. We also calculate a 'UV deficit', which is
the factor by which the Comptonisation model overpredicts the
flux in the UVW1 filter if its seed photons put at 0.1 eV rather than
fixed to the disc temperature. Such low energy seed photons may be
produced by cyclo-synchrotron emission by the same thermal electrons
which produce the Compton scattered emission interacting with the
tangled magnetic field in the hot flow itself ([1]};
[2]}; [3]}). Fig. REF  shows how these evolutions during
the outburst. This confirms the results seen in the three individual
spectra discussed above. The 'UV excess' (red triangles) increases
strongly with time, as the soft X-ray disc component makes less and
less contribution to the UV flux. The disc is not significantly
detected in the final three spectra, but the trend in UV excess is
already clearly apparent. Conversely, the UV deficit (blue squares)
decreases, and is roughly consistent with the observed UV flux from
spectrum 21 onwards.
